Forming tool

ABSTRACT

A deforming tool having a die and prestress ring for the extrusion of a workpiece. The prestress ring being constructed so that the radial prestress exerted by it on the die in a region of transition from one to the other of two converging inner faces of the die, which together form an internal angle of less than 180 degrees, and/or in the region of a sudden transition from a low to a high value of the pressure exerted by the workpiece during its deformation radially on the die, is less than in the regions adjacent to the transition.

The invention relates to a forming tool having a die and a prestressingring surrounding the die.

Such a forming tool can be used for cold-extrusion. The die optionallyconsists of steel, especially of sintered hard metal.

In a known forming tool of that kind (DE 38 34 996 C2), which is usedfor cold extrusion the die is cylindrical, and the prestress exerted bythe prestressing ring ensures that plasticizing, fatigue or rupture ofthe die as a result of internal excess pressure is avoided.

Dies used for extrusion of workpieces are, however, often polygonal incross-section and axial section, that is to say, they have a transitionfrom one to the other of two converging inner faces of the die that forman angle of less than 180°. In the region of these transitions, as aconsequence of stress concentrations and the repeated cyclic loadingvery high tensile stresses which exceed the yield stress of the diematerial can occur, or cracks and fatigue fractures can appear.

The service life of such a forming tool is accordingly short. It istherefore also known (U.S. Pat. No. 3,810,382) for the die, which isprestressed by an encircling band, to be constituted by severalindividual parts. Such a solution is expensive, however.

Such cracks and fatigue fractures can also appear in a region of the diein which its stress by radial pressure during deformation of theworkpiece changes abruptly from a low value, usually zero, to a highvalue, for example approximately level with the end face of a workpiecelocated in the die on which a press stamp is acting, when the edge ofthis end face is contacted by the inside of the die, even when theinside is cylindrical.

The invention is based on the problem of providing a forming tool of thekind mentioned in the introduction, which, when using a one-part die,can be subjected to relatively high stress at transitions of the saidkind without the risk of the die being destroyed.

According to the invention, this problem is solved in that theprestressing ring is constructed so that the radial prestress exerted byit on the die in the region of a transition from one to the other of twoconverging inner faces of the die, which together form an internal angleof less than 180°, and/or in the region of a sudden transition from alow to a high value of the pressure exerted by the workpiece during itsdeformation radially on the die, is less than in the regions adjacent tothe transition.

In this solution a flexural prestress is produced around thetransitions. This counteracts the formation of cracks.

With a cylindrical die or a cylindrical prestressing ring, theconnection of the two can be effected by thermal shrinking, in that theprestressing ring is heated or the die is cooled and the prestressingring is then pushed onto the die.

Preferably, the engagement surfaces of prestressing ring and die areconical. This facilitates joining of the same by axial pressing, so thata force fit is obtained.

Preferably, at least one of the two engagement surfaces of prestressingring and die is machined in accordance with a desired prestressdistribution. Instead of that, or in addition thereto, the materialproperties of the die and/or prestressing ring can be selected inaccordance with a desired prestress distribution. This enables theradial forces in the region of the transitions to be matched in anoptimum manner.

Using conventional shrink rings of solid steel, it is not in practicepossible to achieve a radial prestress distribution with a modificationof more than 10 to 15%.

By dividing the prestressing ring into at least two concentric rings,however, these values can be increased. At least one of the prestressingrings can therefore be surrounded by a reinforcing band. This lengthensthe service life of the prestressing ring. In addition, the entirereinforcing is strengthened from 50 to 70%, and consequently an increasein the modification of the prestress distribution is possible, so thatits maximum value can be around 75 to 125% of the minimum value.

Thus, at least one of the two engagement surfaces of prestressing ringand reinforcing band can be machined in accordance with the desiredprestress distribution.

Furthermore, it is possible to machine the inner or outer side of anintermediate tube, which is arranged between the die and theprestressing ring, or the outside of the die adjacent to theintermediate tube in accordance with the desired prestress distribution.This intermediate tube is able to reduce harmful influences on the diecaused by high forces occurring during assembly and in operation.

In all cases, the engagement surfaces can be superfinished, for examplepolished. This enables the desired prestress distribution to be veryexactly configured.

It is thus possible to ensure that the prestressing ring has regions ofalternating material properties. In particular, the prestressing ringcan consist of several rings of different material rigidity and/ordifferent radial dimensions. It is thus possible to configure thedesired prestress distribution in a simple manner.

The invention and its developments are described in detail hereinafterwith reference to drawings of preferred embodiments, in which

FIGS. 1 to 3 show diagrammatically in axial section differentembodiments of the invention in their application to dies havingdifferent inner contours,

FIG. 4 is a diagrammatic illustration, in axial section, of a formingtool according to the invention for explaining the inventive concept,

FIGS. 5 to 13 show, in axial section, different embodiments of formingtools according to the invention, and

FIGS. 14 to 16 show developed views of different engagement surfaces ofdie and prestressing ring.

The forming tool shown in FIG. 1 contains a die 1 in a prestressing ring2, the engagement surfaces of die 1 and prestressing ring 2 beingconical. The inside of the die 1 has a transition 3 in the form of acircular edge at which a first circular cylindrical inner face 4 changesinto a conical second inner face 5, the two inner faces 4 and 5 formingan internal angle of less than 180°. The conical inner face 5 thenchanges at a further edge 6 into a third circular cylindrical inner face7, the two inner faces 5 and 7 forming an internal angle of more than180°. The die 1 and the prestressing ring 2 have conical engagementsurfaces of complementary cone angle and are held together with a forcefit.

Such a forming tool can be used to form by cold extrusion a conicallystepped workpiece or a cylindrical workpiece having a diametercorresponding to the diameter of the inner face 7.

The forming tool shown in FIG. 2 differs from that shown in FIG. 1merely in that the inner contour of the die 1a is in the form of arectangle, the inner faces 8 of which converge at transitions 9 formedby the edges at internal angles of 90°.

A round slug which has been compressed by pistons from both end facescan be introduced into the die 1a of such a forming tool. It is possiblein this manner to manufacture polygonal nut components, into which athread is cut.

The forming tool shown in FIG. 3 differs from that shown in FIG. 1likewise merely in the inner contour of the die 1b. Here, the upper partof the inner contour is hexagonal, the inner faces likewise forminginternal angles of less than 180°, in this case 60°, at the transitions9 formed by the edges; the transitions 9 can also be bevelled orrounded. Using this forming tool, workpieces in partially round andpartially hexagonal form can be manufactured by cold extrusion.

FIG. 4 illustrates diagrammatically the basic principle of the inventionin the forming tool shown in FIG. 1. According to that principle, theprestressing ring 2 is designed so that the radial prestress it exertson the die 1, indicated by the arrangement of parallel arrows, in theregion of the transition 3 from one to the other of the two converginginner faces 4, 5 is less than in the regions adjacent to the transition.This generates a flexural prestress, indicated by the two curved arrows11 and 12, around the transition 3, so that as the internal pressure isexerted the critical cross-section is relieved of stress in a planecoinciding with the transition 3, and thus formation of cracks becauseof a stress concentration in this region is counteracted.

The manner in which this varying distribution of the prestress can beachieved is explained hereinafter by the example of FIGS. 5 to 13.

According to FIG. 5, the conical inner face of the prestressing ring 2lying adjacent to the conical outer face of the die 1 has acircumferential groove 13 machined into it, for example, by grinding, inthe region of the transition 3, the radial depth of which groove isgreatest at the level of the transition 3, or rather in the radial planethereof, and which decreases continuously with no transition in an axialdirection towards the edges of the groove 13. In the region of thegroove 13 the prestressing ring 2 therefore lies with less radialpressure against the die 1, so that the prestress it exerts on the dieis lowest in the region of the groove 13 and greatest outside the groove13. The prestressing ring 2 is furthermore enclosed by a reinforcingband 14 in the form of a encircling band of sheet metal which prolongsthe service life of the prestressing ring 2. In this manner a 50 to 70%stronger reinforcement of the die 1 can be achieved, which correspondsto an increase in the desired prestress so that the maximum prestressamounts to 75 to 125% of the minimum value. The reinforcing band 14 isfurther surrounded by an outer ring 15, which forms a housing.

The embodiment shown in FIG. 6 differs from that shown in FIG. 5 merelyin that the groove 13 is machined not in the prestressing ring 2 but inthe outside of the die 1.

The embodiment shown in FIG. 7 differs from that shown in FIG. 6essentially only in that an intermediate tube 16 is shrunk down onto thedie 1 between the die 1 and the prestressing ring 2, the adjacentsurfaces of die 1 and intermediate tube 16 are circular cylindrical, andthe outside of the intermediate tube 16 has the same but complementarycone angle as the adjacent inner face of the prestressing ring 2 andcontains the groove 13. The die 1 can accordingly be constructed withthinner walls.

The embodiment shown in FIG. 8 differs from that shown in FIG. 7 merelyin that the groove 13 is formed not on the outside but on the inside ofthe intermediate tube 16.

The embodiment shown in FIG. 9 differs from that shown in FIGS. 7 and 8merely in that it is not the intermediate tube 16 but the outside of thedie 1 that is provided with the groove 13.

The radial depth of the groove 13 in all examples is only severalhundredths to a few tenths of a millimeter and has been illustrated onan exaggeratedly large scale in the drawings.

The embodiment shown in FIG. 10 differs from that shown in FIG. 5 merelyin that the prestressing ring 2 comprises three axially adjacent regions17, 18 and 19 with alternating material properties, the regions 17 to 19being formed by separate rings of which the two outer rings 17 and 19have a greater rigidity or hardness than the middle region 18. Thedistribution of prestress is therefore similar to that illustrated inFIG. 4.

The embodiment shown in FIG. 11 differs from that shown in FIG. 5 merelyin that the prestressing ring 2 comprises a radially inner region 20 anda radially outer region 21, which have different material properties.The regions 20 and 21 are thus in the form of rings, of which theradially inner ring has a greater material rigidity than the radiallyouter ring, the engagement surfaces of the regions 20 and 21 beingtrapezoidal in cross-section and the longer of the two parallel sides ofthe trapezium lying radially outside and the trapezium beingequal-sided.

The embodiment shown in FIG. 12 differs from that shown in FIG. 10merely in that the outer diameter of the middle region 18 is smallerthan the outer diameter of the embodiment shown in FIG. 10, only abouthalf the size, and the reinforcing band 14 is divided into three axiallyadjacent encircling bands 22, 23 and 24, of which the middle encirclingband 23 has a smaller internal diameter than the two outer encirclingbands 22, 24.

The embodiment shown in FIG. 13 differs from that shown in FIG. 10merely in that the prestressing ring 2 comprises five axially mergingregions 17, 18, 19, 25 and 26, which have alternating materialproperties. The intermediate regions 25 and 26 lying between the axiallyouter regions 17 and 19 on the one hand and the middle region 18 aremore rigid or harder than the middle region 18 but less rigid than theouter regions 17 and 19.

FIG. 14 illustrates the developed view of the inner face of aprestressing ring 2, as can be provided in the case of the embodimentshown in FIG. 3, in the form of a diagram in cartesian coordinates, Δrindicating the deviation of the surface of a circular cylindrical facein a radial direction outwards, z indicating the axial direction of theprestressing ring 2, and ψ indicating the circumferential direction. Asone sees, the prestressing ring 2 has depressions 27 on its radiallyinner surface (which are illustrated as raised areas or humps because ofthe direction of Δr), each of which faces towards one of the corners ofthe die 1b at the intersection of transitions 9 and 10; of the total ofeight depressions 27 in the case of the die 1b, only two areillustrated. Conversely, the areas between the depressions 27represented as "valleys" face towards the transitions 9 of the die 1b.

The surface shown as a developed view in FIG. 15 corresponds to theradially inner surface of the prestressing ring 2 shown in FIG. 5, thegroove 13 being illustrated as a hump (because of the sign of Δr).

The surface of the inside of the prestressing ring 2 illustrated in FIG.16 corresponds to a construction of the die 1a shown in FIG. 2, eachtransition 9 having associated with it a depression 28 (againillustrated as a wave-like hump) extending axially in the z-direction.That is to say, altogether four depressions are provided where the die1a is a rectangular socket, but only two are shown.

It is clear that the surface illustration shown in FIG. 15 also applies,for example, to the die 1 shown in FIG. 6, if the direction of Δr inFIG. 15 is reversed.

I claim:
 1. A forming tool having a die and a prestressing ringsurrounding the die and exerting radial prestress on the die, in whichthe prestressing ring is constructed so that the radial prestressexerted by it on the die, in at least one of a region of a transitionfrom one to another of two converging inner faces of the die, whichtogether form an internal angle of less than 180° and a region of asudden transition from a low to a high value of pressure exerted by aworkpiece during its deformation radially on the die, is less than inregions adjacent to the transition.
 2. A forming tool according to claim1, in which engagement surfaces of the prestressing ring and die areconical.
 3. A forming tool according to claim 2, in which at least oneof the two engagement surfaces of the prestressing ring and die ismachined to have a predetermined prestress distribution.
 4. A formingtool according to claim 1, in which material properties of at least oneof the die and the prestressing ring are selected to have apredetermined prestress distribution.
 5. A forming tool according toclaim 1, in which the prestressing ring consists of at least twoconcentric rings.
 6. A forming tool according to claim 1 in which theprestressing ring is surrounded by a reinforcing band.
 7. A forming toolaccording to claim 6, in which at least one of two engagement surfacesof the prestressing ring and reinforcing band is machined to have apredetermined prestress distribution.
 8. A forming tool according toclaim 1, including an intermediate tube between the die and theprestressing ring.
 9. A forming tool according to claim 8, in which oneof an inner and outer side of the intermediate tube is machined to havea predetermined prestress distribution.
 10. A forming tool according toclaim 8, in which an outside surface of the die adjacent theintermediate tube is machined to have a predetermined prestressdistribution.
 11. A forming tool according to claim 3, in which theengagement surface is superfinished.
 12. A forming tool according toclaim 1, in which the prestressing ring has regions of alternatingmaterial properties.
 13. A forming tool according to claim 12, in whichthe prestressing ring consists of several rings having at least one of adifferent material rigidity and different radial dimensions.